Within the circadian (24-hour) cycle, a person usually sleeps approximately 8 hours and is awake 16. During the wakeful hours, mental and physical functions are most active and tissue cell growth increases. During sleep, voluntary muscle activities nearly disappear and there is a decrease in metabolic rate, respiration, heart rate, body temperature, and blood pressure. The activity of the digestive system increases during the resting period, but that of the urinary system decreases. Hormones secreted by the body, such as the stimulant epinephrine (adrenaline), are released in maximal amounts about two hours before awakening so that the body is prepared for activity.

The circadian cycle is controlled by a region of the brain known as the hypothalamus, which is the master centre for integrating rhythmic information and establishing sleep patterns. A part of the hypothalamus called the suprachiasmatic nucleus (SCN) receives signals about light and dark from the retina of the eye. Upon activation by light, special photoreceptor cells in the retina transmit signals to the SCN via neurons of the retinohypothalamic tract. The signals are further transmitted to the pineal gland, a small cone-shaped structure that is attached to the posterior end (behind the hypothalamus) of the third cerebral ventricle and that is responsible for the production of a hormone called melatonin. Cyclical fluctuations of melatonin are vital for maintaining a normal circadian rhythm. When the retina detects light, melatonin production is inhibited and wakefulness ensues; light wavelength (colour) and intensity are important factors affecting the extent to which melatonin production is inhibited. In contrast, in response to darkness, melatonin production is increased, and the body begins to prepare for sleep. Sleep-inducing reactions, such as decreases in body temperature and blood pressure, are generated when melatonin binds to receptors in the SCN.

The natural time signal for the circadian pattern is the change from darkness to light. Where daylight patterns are not consistent, as in outer space, regimented cycles are established to simulate the 24-hour day. If one tries to break the circadian rhythm by ignoring sleep for a number of days, psychological disorders begin to arise. The human body can learn to function in cycles ranging between 18 and 28 hours, but any variance greater or less than this usually causes the body to revert to a 24-hour cycle. Even in totally lighted areas such as the subpolar twilight zone, the body has regular cycles of sleep and wakefulness once the initial adjustment has been made.

Any drastic shift in the circadian cycle requires a certain period for readjustment. Each individual reacts to these changes differently. Travel across a number of time zones is commonly accompanied by circadian rhythm stress, sometimes called “jet lag.” For example, jet travel between Tokyo and New York City creates a time difference of 10 hours; it usually takes several days for the body to readjust to the new day-night pattern. Too-frequent shifts in circadian patterns, such as several transoceanic flights a month, can lead to mental and physical fatigue. Preflight or postflight adaptation can be achieved by gradually changing one’s sleeping patterns to simulate those that will be necessary in the new environment. Space travel is even more extreme. Astronauts first encounter rapid changes in the day-night cycle while in Earth’s orbit. Beyond this, the void becomes a constant blackness with no observable distinction between daytime and nighttime.

The circadian cycle can alter the effectiveness of some drugs. For example, the timing of administration of hormonal drugs so as to be in accord with their natural circadian production pattern seems to place less stress on the body and produce more effective medical results.